[llvm-dev] [LLD] Support DWARF64, debug_info "sorting"

Wed Nov 11 00:55:07 PST 2020

On Wed, 11 Nov 2020 at 05:41, David Blaikie <dblaikie at gmail.com> wrote:

> +James for context too (always good to include the folks from the
> original threads for continuity)
>
> Yeah, my general attitude there was just twofold, one that the
> discussion had strayed fairly far from the review (so interested
> parties might not see it, both because it's a targeted review thread
> on the noisy llvm-commits, and because fo the title not having much
> connection to the discussion) and it seemed to be somewhat
> abstract/general - and there's a balance there. "We should do this
> because I need it" (we shouldn't be implementing features for
> especially niche use cases/if they don't generalize) isn't always a
> compelling motivation but "we should do this because someone might
> need it" isn't either (we shouldn't be implementing features that have
> no users).
>
> The major drawback in sorting, is the need to parse DWARF, even a
> little bit of it (only the first 4 bytes of a section to tell which
> version it is - first 12 if you want to be able to jump over
> contributions and check /all/ contributions coming from a given input
> object file (it might contain a combination of DWARFv4 and DWARFv5)
> and then the hairy uncertainty of which sections to check (do you
> check them all? well, all the ones with length prefixes that
> communicate DWARF32/64 - some sections don't
> (debug_ranges/loc/str/macro for instance, if I recall correctly)...
> and if something has some 4 and 5, does it get sorted to the start? I
> guess so.
>
> I assume this comment is meant to say DWARF32/DWARF64, not DWARFv4 and
DWARFv5, as the DWARF version (as opposed to the 32/64 bit style) is
irrelevant to this, I believe, at least for the current known DWARF
standards. Whilst the majority of objects will only have a single CU in
them, there will be exceptions (LTO-generated objects, -r merged objects
etc), so we do need to consider this approach. Mixtures would certainly be
possible, and there's no guarantee the CUs would be in a nice order with
32-bit blocks before 64-bit blocks. If I follow this to its full
conclusion, you could potentially end up with a single .debug_info
(.debug_line, .debug_rnglists etc) input section with a mixture of
DWARF32/DWARF64 sub-sections, which, if following the reordering approach,
the linker might have to split up internally in order to rearrange (aside -
there's some interesting crossover with ideas I've been considering
regarding the Fragmented DWARF topic discussed elsewhere). Maybe the
solution here would be to change producers to produce separate .debug_info
sections containing DWARF32 and DWARF64. This would require other tools,
like llvm-dwarfdump, to be updated too to handle multiple input .debug_info
sections.

I used the -u option more as an example that it might be possible to get
things to work the way we want without needing to have the linker do the
work. The linker currently has a --symbol-ordering-file option which can be
used to request an order for the specified list of symbols. The linker does
this by rearranging the input sections to get as close as it can to the
requested order. We could maybe implement the same on a file/section basis.
It would avoid needing to read the sections themselves, but doesn't solve
the "what to do about mixed single input" case directly (though might allow
the user to dodge the decision at least).

Other ideas I had involved changing the section header properties.
Currently DWARF sections are all SHT_PROGBITS, but we could change that to
e.g. SHT_DWARF_32 or similar, and/or use the sh_info field to contain a
value that would indicate the 32/64 bit nature. I'm not convinced by these
ideas though, as a) I don't know if it translates well to other non-ELF
formats, and b) we can't really control the producers of DWARF at this
stage to conform.

It would be nice if there was a solution that could be consistently applied
across all build systems, linkers and DWARF producers. I don't have one as
yet though.

> On Tue, Nov 10, 2020 at 9:30 PM Eric Christopher via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> >
> >
> > On Wed, Nov 11, 2020 at 12:19 AM Alexander Yermolovich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> This year Igor Kudrin put in a lot of work in enabling DWARF64 support
> in LLVM. At Facebook we are looking into it as one of the options for
> handling debug information over 4gigs in production environment. One
> concern is that due to mix of third party libraries and llvm compiled code
> the final library/binary will have a mix of CU that are DWARF32/64. This is
> supported by DWARF format. With this mix it is possible that even with
> DWARF64 enabled one can still encounter relocation overflows errors in LLD
> if DWARF32 sections happen to be processed towards the end.
> >>
> >> One proposal that was discussed in https://reviews.llvm.org/D87011, is
> to modify LLD linker to arrange debug_info sections so that DWARF32 comes
> first, and DWARF64 after them. This way as long as DWARF32 sections don't
> themselves go over 4gigs, the final binary can contain debug information
> that exceeds 4gig. Which I think will be the common case.
> >>
> >> An alternative approach that was proposed by James Henderson is for
> build system to take care of it, and to use -u to enforce order.
> >
> >
> > +Fangrui Song here for thread visibility
> >
> > Of these two approaches I think that the linker sorting is probably the
> one I'd go with for the reasons you list below - I'm particularly
> sympathetic to not wanting the unintended consequences of using -u here :)
> >
> > I do worry about slowing down general debug links so a "debug info
> sorting" option may make sense, or it may not be worth it after measuring
> the speed difference.
> >
> > Thanks for bringing this up on the list! :)
> >
> > -eric
> >
> >>
> >>
> >> As, I would imagine, most projects of scale are using configurable
> build system that pulls in all the various dependencies automatically in a
> multi-language environment. I think the alternative approach will be more
> fragile than modifying LLD as it relies on a more complex system, and each
> customer of LLD will have to implement this "sorting" in their own build
> systems. The use of -u also kind of abuses this flag, and might have
> unintended consequences. As was pointed out by Wen Lei.
> >> From overhead perspective we only need to access few bytes of DWARF to
> determine if it's 32 or 64 bits. Customers who need DWARF64, already accept
> the overhead that it entails.
> >>
> >> Any thoughts?
> >>
> >> Thank You
> >> Alex
> >> _______________________________________________
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> >
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